The present invention relates to a printed material monitoring apparatus used for various printing machines such as a rotary offset press particularly for detecting defects such as dirt on a printed material.
A printed material monitoring apparatus of conventional type includes a detection sensor which monitors dirt present on a print surface. Such dirt on the print surface is caused by factors such as ink splashing, water dripping or oil dripping and must be always monitored during the time of the printing process. The detection sensor is arranged so as to extend in a direction perpendicular to the direction in which the print surface moves. The detection sensor successively scans the print surface in a linear manner so as to monitor dirt on the print surface.
The detection sensor is constructed in such a manner that charge coupled devices (CCDs) or light reception elements of a photosensor are linearly arranged in measurement points (hereinafter referred to as unit pixel domains) on the print surface to be detected. Each of the CCDs or light reception elements corresponds to each of unit pixel domains. With such detection sensor, since the size of the spaces between lines on which the light reception elements are arranged cannot be less than the size of a light reception element, a resolution is limited to the size of the light reception element. For the reason described above, various printed material monitoring apparatus have been proposed in which light reflected from these unit pixel domains is transmitted to the light reception elements by using optical fibers, whereby a resolution less than the size of the light reception element is obtained (for example, refer to Japanese Patent Laid-open Publication No. 62-11153).
In the described conventional art, however, as the size of the unit pixel domain is subdivided so as to enhance the resolution of detection, the number of light reception elements increases accordingly, so that the length of the detection sensor is augmented. In such a case, the distance between the unit pixel domains and the corresponding light reception elements at the center of the print surface differs from the distance between the unit pixel domains and the corresponding light reception elements on the sides of the print surface. As a result, the length of the optical fibers connecting the unit pixel domains to the light reception elements at the center become short, whereas the length of the optical fibers linking the unit pixel domains to the light reception elements on the sides of the print surface become very long. This results in a problem in that the attenuating factor of the quantity of detected light in the unit pixel domains on both sides of the print surface is increased, thus making it impossible to accurately detect defects such as dirt on the print surface. Especially, when the resolution is enhanced, because of the small area of the unit pixel domain, the quantity of light to be detected is small. The attenuation is such that an optical fiber may have a negative effect on the detection of defects.
Furthermore, in order to perform the process at high speeds, an examination domain is subdivided. However, when the number of unit pixel domains is increased by subdividing an examination domain, the amount of information naturally increases. Therefore, an improvement in resolution and a high-speed process are two problems having opposing solutions. Thus, there has been a demand for realization of an improved printed material monitoring apparatus in which these two problems can be solved.